The present invention relates to a rear bicycle wheel.
FIG. 1 of the annexed drawings illustrates a rear wheel of a known type mounted on the frame of a bicycle 10 having two fork-shaped arms 12 provided at their bottom ends with seats, designated generically by 14, for fixing of a pin defining an axis 16 of transverse rotation. The rear wheel, designated as a whole by 18, comprises a rim 20 having a median plane 22 set in a central position with respect to the arms 12 and the seats 14 for attachment of the frame 10. The wheel 18 comprises a hub 24 that carries a set of sprockets 26 set coaxially with the hub 24. The rim 20 and the hub 24 are connected together by means of one first set of spokes and one second set of spokes, designated, respectively, by 28a and 28b. The spokes 28a of the first set are anchored along one side of the hub 24 adjacent to the sprocket set 26, whilst the spokes 28b of the second set are anchored to the hub 24 on the side of the latter further away from the sprocket set 26.
On account of the encumbrance of the sprocket set 26 in the direction of the transverse axis 16, the spokes 28a, 28b have a different inclination with respect to the median plane 22 of the wheel 18. In FIG. 1, the angles of inclination of the spokes 28a and 28b with respect to the median plane 22 are designated, respectively, by xcex1 and xcex2. The difference between the angles xcex1 and xcex2 is greater, the greater is the axial encumbrance of the sprocket set 26.
In traditional solutions, the fact that the spokes 28a and 28b have a different inclination with respect to the median plane 22 means that the said spokes are subjected to different tensioning forces. In particular, the tensioning force of the spokes 28a will have to be considerably higher than the tensioning force of the spokes 28b in order to keep the rim 20 centred with respect to the median plane 22 of the wheel. If the mean tensioning forces of the spokes 28a and 28b are respectively designated by Fa and Fb, in order to have correct centring of the rim 20 with respect to the median plane 22, the components of these forces in the direction parallel to the axis of rotation of the wheel must be equal and opposite to one another.
The main drawback of the known solution previously described lies in the fact that the spokes of the wheel that are adjacent to the sprocket set undergo a very high tensioning force and are much more subject to the risk of failure as compared to the spokes located on the side of the wheel opposite to the sprocket set.
The purpose of the present invention is to provide a rear bicycle wheel that is free from the aforementioned drawback.
In accordance with the present invention, the above purpose is achieved by a bicycle wheel having the characteristics that form the subject of the main claim.
The characteristics and advantages of the present invention will emerge clearly from the ensuing detailed description, which is provided purely by way of non-limiting example, with reference to the attached drawings in which: